1. Field of the Invention
This invention relates generally to the manufacture of semiconductor devices prepared by a method including photolithography. More particularly, this invention pertains to a method for inspecting semiconductor substrates to determine the completion of stripping (xe2x80x9cendpointxe2x80x9d) during a plasma stripping process to remove a photo-resist material from a semiconductor substrate surface after photolithography.
2. State of the Art
Semiconductor chips are produced in a multi-step process by which a plurality of identical electronic circuits is typically formed on a semiconductor substrate, such as a silicon wafer. The semiconductor substrate is then subdivided (diced) into individual chips which are further processed into packaged semiconductor devices or otherwise secured in higher-level packaging for ultimate use.
The electronic circuits are generally patterned into a semiconductor substrate by a series of steps including photolithography. To elaborate, a photo-resist material is coated onto the semiconductor substrate surface. As disclosed in commonly owned U.S. Pat. No. 5,350,236, issued Sep. 27, 1994, hereby incorporated herein by reference, the temperature of a semiconductor substrate during the application of a material can be monitored by measuring light reflected from a surface of the semiconductor substrate, such that the material and semiconductor substrate are not overheated.
After the photo-resist material has been coated on the semiconductor substrate surface, it is selectively exposed to a radiation source, such as by the passage of radiation (i.e., light, e-beam, or X-rays) through a mask having a desired aperture pattern defined therein. If a positive photo-resist material is used, the exposure to the radiation source converts the positive photo-resist material to a more soluble state which allows the exposed positive photo-resist to be removed with a solvent, thereby leaving a pattern substantially identical to the mask. If a negative photo-resist material is used, the exposure to the radiation source converts the negative photo-resist material to a less soluble state which allows the unexposed positive photo-resist to be removed with a solvent, thereby leaving a pattern substantially identical to the openings in the mask. Whether a positive or a negative photo-resist material is used, the photolithographic process results in a photo-resist pattern which will become the electronic circuit pattern on a semiconductor substrate.
Following the removal of the portions of the photo-resist material in the development process, the semiconductor substrate is subjected to further processing steps which may include doping, etching, and/or deposition of conductive materials in unprotected areas, i.e., areas devoid of photo-resist material. After one or more of these processing steps, the semiconductor substrate is subjected to a stripping step to remove the photo-resist material remaining on the semiconductor substrate.
The stripping of photo-resist material is commonly achieved using plasma etching. In plasma etching, a glow discharge is used to produce at least one reactive species, such as atoms, radicals, and/or ions, from relatively inert gas molecules. Basically, a plasma etching process comprises 1) at least one reactive species is generated in a plasma from a bulk gas, 2) the reactive species diffuses to a surface of a material being etched, 3) the reactive species is absorbed on the surface of the material being etched, 4) a chemical reaction occurs which results in the formation of at least one volatile by-product, 5) the by-product is desorbed from the surface of the material being etched, and 6) the desorbed by-product diffuses into the bulk gas. The materials used as photo-resist are generally organic polymers, such as phenol-formaldehyde, polyisoprene, poly-methyl methacrylate, poly-methyl isopropenyl ketone, poly-butene-1-sulfone, poly-trifluoroethyl chloroacrylate, and the like. Such photo-resist materials are generally etched in plasmas containing pure oxygen to produce species that attack the organic materials to form CO, CO2, and H2O as volatile by-products.
After the removal of the photo-resist material, a subsequent processing step may include heating the semiconductor substrate in a diffusion furnace or applying a layer of material with a chemical vapor deposition system. Occasionally, a semiconductor substrate is inadvertently passed to a thermal furnace or vapor deposition system with incomplete removal of the photo-resist material. The resulting damage to the processing equipment may be severe. For example, furnace diffusion tubes are irreparably damaged by vaporized hydrocarbons and carbon from the photo-resist material and, thus, the furnace diffusion tubes must be replaced. The replacement equipment and/or the downtime to repair the processing equipment is usually very costly.
Furthermore, the photo-resist carrying semiconductor substrate and one or more subsequent semiconductor substrates entering the processing equipment prior to shutdown of the equipment are usually also contaminated and must be discarded. At a late stage of manufacture, a semiconductor substrate may have a value between about $10,000 and $20,000. Thus, even an occasional loss is significant.
Therefore, it is very important that completion (xe2x80x9cendpointxe2x80x9d) of the photo-resist stripping be accurately detected. A common endpoint detection method with plasma etching is disclosed in U.S. Pat. No. 4,377,436, issued Mar. 22, 1983 to Donnelly et al., wherein endpoint detection during plasma-assisted etching is signaled by cessation or onset of spatially confined luminescence resulting from an etch reaction product. The light source for the luminescence comes from the plasma generation. However, as the use of microwave plasma etching has developed, the generation of the plasma has been removed from the etching chamber. The removal of the plasma generation from the etching chamber prevents excess heat buildup in the etching chamber caused by the plasma generation and allows for different frequencies and wavelengths to be used to create free radicals (i.e., the reactive species).
The reactive species is formed remotely in a microwave reaction chamber and transported to the etching chamber, such as shown in U.S. Pat. No. 5,489,362, issued Feb. 6, 1996 to Steinhardt et al. No plasma is present in the stripping chamber with such a microwave plasma system. Therefore, there is no light source present in the chamber that can be used for detection of the endpoint removal of the photo-resist material.
Therefore, it would be advantageous to develop an apparatus and method of luminescent endpoint detection for the stripping of materials in a microwave plasma etching system employing a plasma chamber separate from its etching chamber.
The present invention is an automated method and apparatus for determining the endpoint of the removal of a photo-resist material on the surface of a semiconductor substrate by the detection of fluorescence, reflection, or absorption of light by the photo-resist material. Hereinafter, the term xe2x80x9cemanated lightxe2x80x9d is defined as the light resulting from a light striking the photo-resist material or other material including fluoresced light, reflected light, or absorbed light.
As mentioned above, photo-resist materials are generally organic polymers, such as phenol-formaldehyde, polyisoprene, poly-methyl methacrylate, poly-methyl isopropenyl ketone, poly-butene-1-sulfone, poly-trifluoroethyl chloroacrylate, and the like. Organic substances can generally fluoresce (luminescence that is caused by the absorption of radiation at one wavelength followed by nearly immediate re-radiation at a different wavelength) or will absorb or reflect light. Fluorescence of the photo-resist material at a particular wavelength, or reflection/absorption by the photo-resist material of light at a given wavelength, may be detected and measured, provided the material differs from the underlying semiconductor substrate in fluorescence or reflection/absorption at a selected wavelength or wavelengths. For example, a positive photo-resist generally fluoresces red or red-orange and a negative photo-resist generally fluoresces yellow.
In a particular application of the invention, the presence of photo-resist material on a semiconductor substrate surface may be rapidly and automatically determined, recorded, and used to determine when the photo-resist material has been removed from the semiconductor substrate surface. In a preferred application of the present invention, a semiconductor substrate is introduced into a stripping chamber which receives at least one reactive species, usually generated from oxygen, from a microwave plasma generator. The stripping chamber includes a first optical port and a second optical port positioned in a wall of the stripping chamber. A beam of light from a lamp passes through the first port, strikes the photo-resist material on the semiconductor substrate and is reflected as an emanated beam at an angle through the second optical port. Preferably, the photo-resist material differs from the semiconductor substrate in fluorescence, absorption, and/or reflection properties at some wavelengths of incident light.
The intensity of the emanated light will decrease when the photo-resist is stripped away. When the intensity has decreased to a level indicating that the photo-resist has been completely stripped away, the stripping process can be terminated. This detection method also allows the system to generate an error signal if the level indicating that the photoresist has been stripped is not reached within a certain amount of time. Such an error signal would indicate that a semiconductor substrate was stripping poorly (i.e., too slowly) or the stripping equipment was not functioning properly. This error signal allows for the culling of the offending semiconductor substrate for rework or allows for the stripping equipment to be shut down for repair, which prevents the spread of photo-resist material contamination throughout other process steps. Furthermore, the throughput of the stripping equipment can be increased because empirically established finite strip times used in conjunction with endpoint detection of the photo-resist removal prevents the need for exaggerated strip times to ensure complete stripping.
In this invention, the semiconductor substrate is irradiated with light, which light may be monochromatic, multichromatic, or white. In one variation, the intensity of generated fluorescence particular to the photo-resist material at a given wavelength is measured. In another variation, the intensity is measured at a wavelength which is largely or essentially fully absorbed by the photo-resist material. In a further variation, the intensity of reflected light is measured at a particular wavelength highly reflected by the photo-resist material but absorbed by the substrate.
The intensity of the emanated light is measured by a sensing apparatus and the result inputted to a logic circuit, e.g., a programmable computer. The result may be recorded and used for a decision making step or to activate a culling device.